Analysis of amino acid copolymer compositions

ABSTRACT

Methods for analyzing, selecting, characterizing or classifying compositions of a co-polymer, e.g., glatiramer acetate are described. The methods entail analysis of pyro-glutamate in the composition, and, in some methods, comparing the amount of pyro-glutamate present in a composition to a reference standard.

RELATED APPLICATION INFORMATION

This application is a continuation of U.S. patent application Ser. No. 14/941,006, filed Nov. 13, 2015, which is a continuation of U.S. patent application Ser. No. 14/795,867, filed Jul. 9, 2015, which is a continuation of U.S. patent application Ser. No. 14/019,119, filed Sep. 5, 2013, which is a continuation of U.S. patent application Ser. No. 13/709,586, filed Dec. 10, 2012, which is a continuation of U.S. patent application Ser. No. 12/408,058, filed Mar. 20, 2009, and claims priority to U.S. Provisional Application Ser. No. 61/045,465, filed Apr. 16, 2008. The entire disclosures of the prior applications are considered part of, and are incorporated by reference in their entireties in, the disclosure of this application.

BACKGROUND

Glatiramer acetate (also known as copolymer-1 and marketed as the active ingredient in COPAXONE® by Teva Pharmaceutical Industries Ltd., Israel) is used in the treatment of the relapsing-remitting form of multiple sclerosis (RRMS). According to the COPAXONE® product label, glatiramer acetate (GA) consists of the acetate salts of synthetic polypeptides, containing four naturally occurring amino acids: L-glutamic acid, L-alanine, L-tyrosine, and L-lysine with a reported average molar fraction of 0.141, 0.427, 0.095, and 0.338, respectively. Chemically, glatiramer acetate is designated L-glutamic acid polymer with L-alanine, L-lysine and L-tyrosine, acetate (salt). Its structural formula is:

(Glu, Ala, Lys, Tyr)_(x).xCH₃COOH

(C₅H₉NO₄.C₃H₇NO₂.C₆H₁₄N₂O₂.C₉H₁₁NO₃)_(x).xC₂H₄O₂  CAS—147245-92-9

SUMMARY OF THE INVENTION

The invention is based, at least in part, on the identification and characterization of L-pyroGlutamic Acid (pyro-Glu) as a structural signature of glatiramer acetate (GA). Analysis of this signature component of GA is useful to assess product and process quality in the manufacture of GA.

Described herein is a method of selecting a batch of a composition comprising an amino acid copolymer (e.g., GA), the method comprising: providing a batch of a composition comprising an amino acid copolymer; measuring the amount of pyro-glutamate (pyro-Glu) in the batch; and selecting the batch if the amount of pyro-Glu in the batch is within a predetermined range. In this method, as in the other methods described herein, the measuring step can employ any suitable method and the units used to express the measured amount of pyro-Glu can be any suitable units (e.g., ppm or mole percent of chains). In measuring the amount of pyro-Glu, one can, e.g., measure the concentration of pyroGlu in a sample or the total amount of pyro-Glu in sample. However an amount of pyro-Glu is measured and whatever units are used to express the measured amount, the concentration of pyro-Glu in the selected batch is between 2000 and 7000 ppm (in some cases between 2500 and 6500 ppm) on a dry weight/dry weight basis.

Also described is a method for preparing a pharmaceutical composition comprising: providing a batch of a composition comprising an amino acid copolymer, measuring the amount of pyro-Glu in the batch; and preparing a pharmaceutical composition comprising at least a portion of the batch if the amount of pyro-Glu in the batch is within a predetermined range. Here too, the measuring step can employ any suitable method and units used to express the measured amount of pyro-Glu can be any suitable units (e.g., ppm or mole percent of chains). However the pyro-Glu is measured and whatever units are used to express the measured amount, the concentration of pyro-Glu in the selected batch is between 2000 and 7000 ppm (in some cases between 2500 and 6500 ppm) on a dry weight/dry weight basis.

A batch of a composition comprising an amino acid copolymer can be all or part of the product of a copolymer manufacturing process (e.g., all or part of a single manufacturing run). In some cases, one batch is analyzed. In some cases two or more batches are analyzed. In some cases, multiple samples taken from a single batch are analyzed. The composition containing a copolymer can be a drug substance (DS) (also known as an active pharmaceutical ingredient (API)), a drug product (DP), or a process intermediate. The copolymer can be glatiramer acetate.

Also described is a method for preparing a pharmaceutical composition comprising glatiramer acetate, comprising: polymerizing N-carboxy anhydrides of L-alanine, benzyl-protected L-glutamic acid, trifluoroacetic acid (TFA) protected L-lysine and L-tyrosine to generate a protected copolymer; treating the protected copolymer to partially depolymerize the protected copolymer, deprotect benzyl protected groups and deprotect TFA-protected lysines to generate glatiramer acetate; and purifying the glatiramer acetate, wherein the improvement comprises: measuring the amount of pyro-glutamate (pyro-Glu) in the purified glatiramer acetate. In other embodiments the improvement further comprises selecting the purified glatiramer acetate for use in the preparation of a pharmaceutical composition if the amount of pyro-Glu in the purified glatiramer acetate is within a predetermined range. In some embodiments the concentration of pyro-Glu in the selected purified glatiramer acetate 2000-7000 ppm or 2500-6500 ppm.

Also described is a method for preparing a pharmaceutical composition comprising glatiramer acetate, the method comprising: polymerizing N-carboxy anhydrides of L-alanine, benzyl-protected L-glutamic acid, trifluoroacetic acid (TFA) protected L-lysine and L-tyrosine to generate a protected copolymer; treating the protected copolymer to partially depolymerize the protected copolymer and deprotect benzyl protected groups and deprotecting TFA-protected lysines to generate glatiramer acetate; and purifying the glatiramer acetate, wherein the improvement comprises: measuring the amount of pyro-glutamate (pyro-Glu) during or after the polymerizing step.

Described herein is a method for preparing a pharmaceutical composition comprising glatiramer acetate, the method comprising: polymerizing N-carboxy anhydrides of L-alanine, benzyl-protected L-glutamic acid, trifluoroacetic acid (TFA) protected L-lysine and L-tyrosine to generate a protected copolymer; treating the protected copolymer to partially depolymerize the protected copolymer and deprotect benzyl protected groups and deprotecting TFA-protected lysines to generate glatiramer acetate; and purifying the glatiramer acetate, wherein the improvement comprises: measuring the amount of pyro-glutamate (pyro-Glu) during or after the partial depolymerization step.

In the aforementioned methods for preparing a pharmaceutical composition the improvement can further comprise: measuring the amount of pyro-glutamate (pyro-Glu) in the purified glatiramer acetate; selecting the purified glatiramer acetate for use in the preparation of a pharmaceutical composition if the amount of pyro-Glu in the purified glatiramer acetate is within a predetermined range; and preparing a pharmaceutical composition comprising at least a portion of the selected purified glatiramer acetate. In various embodiments, concentration of pyro-Glu in the selected purified glatiramer acetate is, for example, 2000-7000 ppm or 2500-6500 ppm.

Also described is a method for preparing a pharmaceutical composition comprising glatiramer acetate, comprising: polymerizing N-carboxy anhydrides of L-alanine, benzyl-protected L-glutamic acid, trifluoroacetic acid (TFA) protected L-lysine and L-tyrosine to generate a protected copolymer; treating the protected copolymer to partially depolymerize the protected copolymer and deprotect benzyl protected groups and deprotecting TFA-protected lysines to generate glatiramer acetate; and purifying the glatiramer acetate, wherein the improvement comprises: measuring the amount of benzyl alcohol during or after the polymerizing step, wherein the amount of benzyl alcohol is correlated to the amount of pyro-Glu.

Described herein is a method for preparing a pharmaceutical composition comprising glatiramer acetate, comprising: polymerizing N-carboxy anhydrides of L-alanine, benzyl-protected L-glutamic acid, trifluoroacetic acid (TFA) protected L-lysine and L-tyrosine to generate a protected copolymer; treating the protected copolymer to partially depolymerize the protected copolymer and deprotect benzyl protected groups and deprotecting TFA-protected lysines to generate glatiramer acetate; and purifying the glatiramer acetate, wherein the improvement comprises: measuring the amount of benzyl alcohol during or after the partial depolymerization step, wherein the amount of benzyl alcohol is correlated to the amount of pyro-Glu.

In either of the methods entailing measuring the amount of benzyl alcohol, the improvement can further comprise: measuring the amount of pyro-glutamate (pyro-Glu) in the purified glatiramer acetate; selecting the purified glatiramer acetate for use in the preparation of a pharmaceutical composition if the amount of pyro-Glu in the purified glatiramer acetate is within a predetermined range; and preparing a pharmaceutical composition comprising at least a portion of the selected purified glatiramer acetate. In various embodiments, the concentration of pyro-Glu in the selected purified glatiramer acetate is, for example, 2000-7000 ppm or 2500-6500 ppm.

The step of measuring the amount of pyro-Glu in a batch or sample can include any method for measuring (qualitatively or quantitatively) the amount of pyro-Glu and can include multiple steps and processes. Thus, the measuring step can include, for example: direct measurement of the copolymer, size fractionating the copolymer, digesting the copolymer, or cleaving the copolymer. The measuring can be based on, for example, the total amount of pyro-Glu or on the concentration of pyro-Glu or on the percentage of copolymer peptides that include a pyro-Glu. The measured amount can be expressed in any convenient units, e.g., in weight, weight percent or ppm (all measured in dry weight, i.e., total dry weight pyro-Glu in the sample/total dry weight of the sample), or mole percent of peptide chains. It should be noted that as the mole percent of chains and weight percent of chains are related by the average molecular weight of the copolymer, it is possible to interconvert between these values if the average molecular weight is known, estimated or assumed. However, the precise value of the calculated mole percent of chains will depend on whether the average molecular weight value used is a number average molecular weight (Mn), weight average molecular weight (Mw) or peak average molecular weight (Mp). While Mw, Mp or Mn can be used in the calculations, it is preferable to use Mn. Whatever method is used to measure pyro-Glu in the batch or sample, and whatever units are used to express the measured pyro-Glu in the batch or sample, the concentration of pyro-Glu in the selected batch is between 2000 and 7000 ppm (mass_(pyro-Gly)/mass_(total))×10⁶).

The methods can also include selecting the batch or pharmaceutical preparation as suitable for sale or administration to a human when the concentration of pyro-Glu in the batch is within a predetermined range, e.g., 2000-7000 ppm.

The measuring step can comprise providing a value (e.g., in units such as ppm, percent of peptide chains) for the amount of pyro-Glu in the batch and optionally comparing the value to a reference value (e.g., a specification for commercial release of a copolymer product).

Where the value for the amount of pyro-Glu in a batch of glatiramer acetate has a preselected relationship with the reference value, the method can include classifying, selecting, accepting, discarding, releasing, or withholding a batch of glatiramer acetate; reprocessing a batch through a previous manufacturing step; processing a batch of glatiramer acetate into drug product, shipping the product from a batch of glatiramer acetate, moving the batch of glatiramer acetate to a new location; or formulating, labeling, packaging, selling, offering for sell, or releasing a batch of glatiramer acetate into commerce.

Also described herein is a method of analyzing a composition comprising glatiramer acetate for the presence or amount of pyro-Glu, the method comprising: digesting a sample of the composition with a peptidase or protease (e.g., pyroglutamate amino peptidase, an endopeptidase, and trypsin), comparing the digestion products to a pyro-Glu reference standard, and evaluating the amount of pyro-Glu in the sample relative to the reference standard, thereby analyzing a composition comprising glatiramer acetate. In some cases the digestion products are separated by a chromatographic process prior to comparing the digestion to a pyro-Glu reference standard. Thus, the comparison step can include a chromatographic method (e.g., liquid chromatography, particularly HPLC) to separate components and mass spectrometry (MS) analysis or UV absorbance analysis to detect the amount of various components.

In some cases the step of measuring pyro-Glu in the batch comprises: digesting a sample with a peptidase or a protease; isolating pyro-Glu present in the digested sample; and measuring the amount of isolated pyro-Glu. The isolating step can comprise a chromatographic method (e.g., liquid chromatography, particularly HPLC). The measuring step can comprise mass spectrometry (MS) analysis or UV absorbance analysis

The measuring step can comprise measuring UV absorbance (e.g., at 180-250 nm, 200 nm, or 210 nm). The isolating step can comprise a chromatographic method (e.g., liquid chromatography, particularly HPLC). The determining step can comprise mass spectrometry (MS) analysis. The isolating step can comprise HPLC and the measuring step can comprise UV absorbance analysis. The isolating step can comprise liquid chromatography and the measuring step can comprise mass spectrometry (MS) analysis.

In some cases, the pyro-Glu content of the copolymer or glatiramer acetate preparation is between 2000 to 7000 ppm, e.g., between 2500-6500 ppm, e.g., between 3000-6000 ppm, e.g., between 3300-4400 ppm. In some cases, the pyro-Glu content of the copolymer or glatiramer acetate preparation is less than 7000 ppm, e.g., less than 6000 ppm, less than 5000 ppm, less than 4000 ppm, less than 3000 ppm, or less than 2000 ppm.

As used herein, a “copolymer”, “amino acid copolymer” or “amino acid copolymer preparation” is a heterogeneous mixture of polypeptides comprising a defined plurality of different amino acids (typically between 2-10, e.g., between 3-6, different amino acids). A copolymer may be prepared from the polymerization of individual amino acids. The term “amino acid” is not limited to naturally occurring amino acids, but can include amino acid derivatives and/or amino acid analogs. For example, in an amino acid copolymer comprising tyrosine amino acids, one or more of the amino acids can be a homotyrosine. Further, an amino acid copolymer having one or more non-peptide or peptidomimetic bonds between two adjacent residues is included within this definition. A copolymer is non-uniform with respect to the molecular weight of each species of polypeptide within the mixture.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows release of alanine from dipeptides upon HBr/acetic acid treatment. A=Ala=Alanine; E=Glutamic Acid; K=Lysine; Y=Tyrosine. All dipeptides were prepared at a concentration of 10 mM. Two dipeptides (A-A-NH2 and A-Y-NH2) were amidated at the C-terminus.

FIG. 2 is an LC-MS trace showing an unusual amino acid with residual mass of 111 Da (“X”) at the N-terminus of a peptide derived from trypsin-digested Copaxone®. Lys=Lysine; Ala=Alanine.

FIG. 3 shows the structure of L-pyro Glutamic Acid (pyro-Glu) Glatiramer Acetate (GA).

DETAILED DESCRIPTION OF THE INVENTION

Other than molecular weight and amino acid composition, which are specified in the approved label for the product, the label and other available literature for Copaxone® does not provide detailed information about the physiochemical characteristics of the product. Based on detailed characterization of the product and process kinetics, the inventors have unexpectedly found a signature component of GA, L-pyro-Glutamic Acid (pyro-Glu) GA, that can be evaluated to assess the GA manufacturing process and product quality. In particular, evaluation of pyro-Glu content can identify differences in materials that are not observed by looking at molar mass and amino acid composition alone. By evaluating the pyro-Glu content of a sample of a copolymer, e.g., GA, one can identify non-conforming copolymer compositions. Accordingly, pyro-Glu content can be used to evaluate product and process quality for GA.

The production of GA entails both polymerization of amino acids and partial depolymerization of the resulting peptides. It has now been found that depolymerization is highly specific and non-stochastic and occurs to a disproportionately high extent to the N-terminal side of glutamate residues. Indirectly, this results in pyro-Glu GA as a signature structural characteristic of GA, surprisingly occurring primarily as a consequence of depolymerization. Pyro-Glu is present in GA in a range of 2000-7000 ppm and can be assessed to identify or evaluate GA and its method of manufacture, and/or to evaluate the quality or suitability of a GA product for pharmaceutical use.

Methods for Manufacture of Glatiramer Acetate

Generally, the process for the manufacture of glatiramer acetate includes three steps:

Step (1): polymerization of N-carboxy anhydrides of L-alanine, benzyl-protected L-glutamic acid, trifluoroacetic acid (TFA) protected L-lysine and L-tyrosine (collectively referred to as NCAs) to result in a protected copolymer,

Step (2): depolymerization and benzyl deprotection of the protected copolymer using hydrobromic acid in acetic acid, and

Step (3): deprotection of the TFA-protected lysines on the product copolymers followed by purification and drying of the isolated drug substance.

In Step (1) of the manufacturing method, the NCAs are co-polymerized in a predetermined ratio using diethylamine as an initiator. Upon consumption of the NCA components, the reaction mixture is quenched in water. The resulting protected polymer (Intermediate-1) is isolated and dried. In Step (2), the protected polymer (Intermediate-1) is treated with anhydrous 33% HBr in acetic acid (HBr/AcOH). This results in the cleavage of the benzyl protecting group on the glutamic acid as well as cleavage of peptide bonds throughout the polymer, resulting in a partially depolymerized product (Intermediate-2) with a reduced molecular weight relative to the parent Intermediate-1 polymer. After the reaction is quenched with cold water, the product polymer is isolated by filtration and washed with water. The Intermediate-2 material is dried before proceeding to Step (3). In Step (3), Intermediate-2 is treated with aqueous piperidine to remove the trifluoroacetyl group on the lysine. The resulting copolymer (Intermediate-3) is subsequently purified using diafiltration/ultrafiltration and the resulting acetate salt dried to produce Glatiramer Acetate drug substance.

Methods for the manufacture of glatiramer acetate have been described in the following publications: U.S. Pat. No. 3,849,550; WO 95/031990 and US 2007-0021324.

Process Chemistry of Synthetic Method and Structural Characterization of GA

By studying the polymerization/depolymerization chemistry using model peptide compounds to model the synthetic process for producing GA, the inventors have found that there are certain rules associated with the chemistry. By developing an understanding of these rules, it is apparent that GA is not a stochastic, or random, mixture of peptides. Rather, there are certain attributes that are conserved from batch-to-batch and can be measured in order to monitor and evaluate process and batch quality.

Specifically, study of the kinetics of the depolymerization step of the GA manufacturing process using model peptide compounds revealed that Step 2 depolymerization occurs to disproportionately high levels on the N-terminal side of glutamate residues. In model compounds, the only appreciable cleavage was on the N-terminal side of glutamate residues (FIG. 1). In the manufacturing process of Glatiramer Acetate, cleavage occurs at all residues, but with a bias towards the N-terminal side of glutamate residues. Further, a modified amino acid, identified as pyro-glutamic acid (pyro-Glu), was found in tryptic peptides of Copaxone® samples. Analysis of aliquots removed from the depolymerization step at various time points and then further processed to produce GA revealed that the amount of pyro-Glu at amino termini increases as the depolymerization time increases. Thus, the level of pyro-Glu in the final GA product is surprisingly primarily a consequence of the depolymerization kinetics and is not accounted for solely by the polymerization chemistry. From this understanding of the chemistry of GA synthesis, and from characterization of the resulting product, it has thus been discovered that pyro-Glu is a signature structural characteristic of glatiramer acetate. The formation of pyro-Glu results from: (1) parameters relating to the polymerization reaction, as well as, surprisingly and unexpectedly, (2) parameters related to the de-polymerization reaction. Accordingly, pyro-Glu can be evaluated and monitored in the manufacture of GA (including in the final drug substance or drug product) in order to, e.g., (i) identify GA, (ii) assess the quality of GA (e.g., of a GA batch), and/or (iii) assess or confirm the quality of the GA manufacturing process.

Methods of Measuring pyro-Glu

Because pyro-Glu is formed during the GA manufacturing process, its presence and level provide useful information regarding GA chemistry and product quality.

Certain methods are described herein for measuring pyro-Glu content in a composition that includes GA. However, it is understood that other methods to measure pyro-Glu can also be used.

One analytical method developed and described herein for the measurement of pyro-Glu content is based on enzymatic cleavage of an N-terminal pyroglutamate residue using pyroglutamate aminopeptidase (e.g., from thermophilic archaebacteria, Pyrococcus furiosus). The amount of pyro-Glu in the resulting enzymatic hydrolysate can be analyzed by a suitable technique, such as reverse phase liquid chromatography, to determine the ppm or w/w % of pyro-Glu in a GA sample. This method does not require knowing the mean chain length or average molecular weight of the GA in the composition. Accordingly, ppm or w/w % of pyro-Glu Glu is a preferred expression of the amount of pyro-Glu in a batch or a sample of copolymer, e.g., GA.

Various methods can be used to determine the percentage of peptide chains bearing pyro-Glu in a GA sample. A determination of mole % or percent of chains bearing pyro-Glu requires a determination of average molecular size or mean chain length. Molecular size can be evaluated e.g., by SEC MALLS (size exclusion chromatography with multiple angle laser light scattering). Mean chain length can be computed e.g., by labeling (e.g., with a radioactive or fluorescent label) the free amino termini with a molecule which can be directly quantified. One analytical method developed and described herein for measuring the percentage of peptide chains bearing pyro-Glu involves combining quantitative Edman degradation with enzymatic removal of pyro-Glu. Such an analysis can entail: 1) quantifying the N-terminal amino acids in a sample of GA before treatment to remove pyro-Glu; and 2) quantifying the N-terminal amino acids in a sample of GA after treatment to remove pyro-Glu.

EXAMPLES Example 1 Depolymerization Kinetics of Glatiramer Acetate Method of Manufacture

To investigate the depolymerization kinetics, the reaction of various dipeptide model compounds with HBr/AcOH was investigated. FIG. 1 shows release of alanine from dipeptides upon HBr/acetic acid treatment as performed in Step 2 of the manufacturing process. All dipeptides were prepared at a concentration of 10 mM. Two dipeptides (A-A-NH2 and A-Y-NH2) were amidated at the C-terminus. As shown in FIG. 1, release of alanine was only observed for A-E(OBn), indicating that dipeptides with Glu(OBn) in the C-terminal position demonstrate the most cleavage over the course of 24-48 h reaction times as compared to dipeptides without Glu in the C-terminal position. Thus, depolymerization occurs to an appreciable extent only on the N-terminal side of glutamate residues in these model systems. In the actual manufacturing process for Glatiramer Acetate, cleavage occurs at all residues, but still shows a strong bias for the N-terminal side of glutamate residues.

Example 2 Presence of N-terminal pyro-Glu Structures

Trypsin digestion of Copaxone® followed by LC-MS analysis identified expected peptides containing each of the amino acids A, E, K and Y. In addition, unexpected peptides were also found. An unusual amino acid (m/z 111) with residual mass of 111 Da was observed at the N-terminus of several such unexpected peptides derived from trypsin-digested Copaxone® (labeled as “X”, FIG. 2). From LC-MS/MS analysis it was determined that the unusual amino acid is pyro-Glu, formed by cycling of N-terminal glutamic acid to form pyro-glutamic acid losing a water molecule [111 Da=129 Da (Glutamic acid residue)—18 Da (H2O)]. FIG. 3 shows the structure of L-pyro Glutamic Acid (pyro-Glu) GA.

Example 3 Evaluation of pyro-Glu Content on a Weight Basis

This example describes a method for evaluating pyro-Glu content in a copolymer composition.

An analytical method developed for the pyro-glutamate content assay is based on enzymatic cleavage of a N-terminal pyro-glutamate residue using pyro-glutamate aminopeptidase (from thermophilic archaebacteria, Pyrococcus furiosus). Pyro-glutamate in the resulting enzymatic hydrolysate is isolated by reverse phase liquid chromatography followed by detection at 200 nm using a reference standard curve prepared with known concentrations of L-Pyro-glutamate. Neurotensin (a commercially available polypeptide having 100% pyro-glutamate at the N-terminus) is assayed as a control to ensure the acceptability of the digestion and adequacy of the HPLC separation. The chromatographic analysis is performed using a Waters Atlantis C18 HPLC column and an isocratic mobile phase consisting of 100% Water, adjusted to pH 2.1 with phosphoric acid. Samples and Standards are held at 2-8° C. The peak corresponding to the pyro-glutamate moiety elutes at a retention time of approximately 12 minutes. The direct measure of pyro-glutamate content is on a w/w basis and the results are expressed as ppm (microgram/gram).

Example 4 Evaluation of pyro-Glu Content on a Percentage of Peptide Chains Basis

The percentage of peptide chains in a sample of GA bearing pyro-Glu can be measured as an alternative to measuring the amount of pyro-Glu in a sample of GA. The percentage of peptide chains bearing pyro-Glu can be determined by combining quantitative Edman degradation with enzymatic removal of pyro-Glu. Thus, the analysis entails: 1) quantifying the N-terminal amino acids in a sample of GA before treatment to remove pyro-Glu; and 2) quantifying the N-terminal amino acids in a sample of GA after treatment to remove pyro-Glu.

An Edman degradation reaction was used to quantify the N-terminal amino acids in a sample of GA before and after treatment with pyroglutamate aminopeptidase (PA) to remove pyro-Glu. This reaction was performed manually to avoid quantitative limitations of automatic N-terminal peptide sequencers. The results of this analysis are presented in the table below.

TABLE 1 N-terminal Amino Acid nmol N-terminal amino acid Before PA Treatment After PA Treatment Amino Acid (st. dev) (st. dev) Ala 25.1 (0.6)  51.7 (0.5) Glu 7.8 (0.3) 15.7 (0.1) Lys 9.0 (0.2) 20.2 (0.8) Tyr 6.5 (0.1) 10.5 (0.2) Total 48.4 98.1

As can be seen in Table 1, above, the N-terminal amino acid concentration increased from 48.4 to 98.1 nmol after PA treatment. This is because removal of pyro-Glu permits detection of peptides that could not previously have been detected by Edman degradation. The percentage of chains bearing pyro-Glu can be calculated as follows: % chains capped by pyroglutamate=(Pafter−Pbefore)/Pafter×100%. In this calculation, Pbefore and Pafter are the concentrations of N-terminal amino acids with and without PA treatment, respectively. In this example, 51% of the polymer chains were capped by pyroglutamate.

Example 5 pyro-Glu Content can Distinguish Glatiramer Acetate

Using the method described in Example 3, the pyro-Glu content of commercial Copaxone® was compared to several other copolymer samples. A sample of glatiramer acetate (M-GA) prepared according to the method described in U.S. Pat. No. 3,849,550 was evaluated for pyro-Glu content. Table 2, below, provides the results of the analysis of a number of compositions, this sample conforms to the range found for pyro-Glu content from a sampling of Copaxone® lots, or between 2500-6500 ppm.

TABLE 2 Analysis of Samples Analysis of Samples Amino acid Molecular composition weight (Mp) (avg. molar P-Glu content Sample (Da) fraction)² (ppm) Copaxone ® 5,000-9,000¹ 0.141 L-Glutamic 2500-6500 ppm⁴ acid 0.427 L-alanine 0.095 L-tyrosine 0.338 L-lysine Glatiramer 8407 (conforms) ³ 4900 ppm acetate (conforms) (conforms) sample (M-GA) Deviating 6579 (conforms) ³ 8200 ppm sample A (conforms) (fails) Deviating 4808 (conforms) ³ 7500 ppm sample B (fails) (fails) ¹Molecular weight range specified in Copaxone ® product label and prescription information ²Average molar fraction target specified in Copaxone ® product label and prescription information ³ Conforms relative to specification range based on label target plus allowance for manufacturing and measurement variability ⁴Range is 75%/125% of Copaxone min/max for 30 commercial samples

To test the ability of pyro-Glu content to distinguish glatiramer acetate from non-conforming copolymers, two control copolymers were tested. The control copolymers were made with deliberate and specific deviations in the timing of NCA addition or in the duration of step 2. As shown in Table 1, both deviating samples A and B were outside of the range for pyro-Glu content determined for Copaxone®. Sample A was within the range for Copaxone® molar mass and amino acid composition while Sample B failed molar mass but conformed in amino acid composition. This data shows that evaluation of pyro-Glu content can identify differences in materials and process not observed by looking at molar mass and amino acid composition alone and illustrates the ability of pyro-Glu measurement to identify non-conforming copolymer. Accordingly, pyro-Glu content can be used to evaluate product and process quality for glatiramer acetate. 

What is claimed is:
 1. A method of selecting a batch of a composition comprising an amino acid copolymer, the method comprising: providing a batch of a composition comprising an amino acid copolymer; measuring the amount of pyro-glutamate (pyro-Glu) in the batch; and selecting the batch if the amount of pyro-Glu in the batch is within a predetermined range, thereby selecting a batch of a composition comprising an amino acid copolymer. 